The fate of two major San Francisco transportation projects appears to hang in the balance in Washington, D.C.

Both the Central Subway and the state’s high-speed rail project, which would terminate in San Francisco, have budgets that are heavily reliant on major chunks of taxpayer money coming from Washington, D.C.

The Central Subway has budgeted some $983 million of its $1.5 billion budget — about 65 percent — to come from the feds. Meanwhile the California High-Speed Rail Authority is banking on about $42 billion from federally controlled dollars toward the overall price tag of $68 billion, equal to about 61 percent.

But will federal funding come through?

With both projects there have been signs that lawmakers in Washington may be reluctant to provide the amount of taxpayer money that is being discussed — if any.

Last fall, in a threat to the Central Subway, House Republicans introduced a bill that would have prohibited transit projects from receiving more than half of their funding from Washington. That bill stalled.

Currently, the Central Subway project continues to wait for approval of a $983 million federal funding guarantee under the New Starts program. Municipal Transportation Agency officials had hoped that funding would be finalized in late 2011 or early this year. Adding further funding drama, a pending House transportation bill includes an amendment that could take away future federal money for the project.

California’s bullet train, meanwhile, faced threats from Republicans in both the House and Senate who have expressed resistance to paying for bullet trains anywhere.

Planners of both the Central Subway and high-speed rail expressed confidence that they can still rely on federal money, and are not letting the uncertainty change immediate plans to move forward.

The Central Subway — an extension of the Third Street Muni Metro line that would connect to Chinatown — has already started construction. Workers have begun digging the tunnel that will run 1.7 miles between 4th and King streets to Stockton and Washington streets.

The high-speed rail authority is making plans to start construction on the first section of track this year in the Central Valley. The rail authority has announced meetings in coming weeks where small businesses can meet with the authority to learn how to bid for work.

But some insist that reliance on federal money might be misguided.

“The MTA can’t contend this is a done deal,” said Jerry Cauthen, a co-founder of Save Muni, a rider-based transit group that is opposed to the Central Subway. “It’s an open question right now” whether the money will come through.

The Federal Transportation Authority has already given the Central Subway $92.4 million. But the remainder is still subject to approval from Congress, which has not yet begun its 60-day review period. It is unclear when that review will begin.

A representative of the Federal Transportation Authority, which administers the New Starts money, noted that the Central Subway project “remains in review,” and declined to elaborate.

San Francisco officials downplayed the prolonged review period for the Central Subway, saying it is normal for review periods on big projects to take longer than anticipated.

“It’s making its way through” the process, said spokeswoman Kristen Holland. “We’re still expecting the execution of the full funding grant agreement.”

The high-speed rail project — which has a much longer timeline than San Francisco’s subway — could also face an uphill battle to secure federal money, observers said.

Richard Little, executive director of USC’s Keston Institute for Public Finance and Infrastructure Policy, said the limping economy might be a hindrance to California’s project. The reason, he said, was that with the federal budget under strain, it could be politically difficult to spend money on a project that many contend is too pricey.

“Future federal support for California high-speed rail is highly unlikely regardless of who holds power in D.C., absent dramatic changes in the national and global economic situations,” Little said.

Nevertheless, California’s high-speed train architects said they expect federal dollars to flow in the future.

“With the federal government’s early investment to begin construction of high-speed rail, we are confident that as we build the initial operating segment in the Central Valley, they will be open to additional investment,” said Jeff Morales, CEO of the California High-Speed Rail Authority. Moreover, Morales said, the bullet train’s budget does not require more federal money for the “next several years.”

One group has already come up with a plan for California to consider if federal funding dries up, or comes in much lower than anticipated.

A mix of new road tolls, vehicle license fees, gas taxes, regional general obligation bonds and revenues from the state’s upcoming cap-and-trade auctions could raise the needed money, according to a report from San Francisco Planning and Urban Research Association.

These sources could yield more than $2.7 billion annually, the report said.

“That is not what I hope happens, or think is going to happen, but if we had to, we could,” said Gabriel Metcalf, SPUR director.